(1) Field of the Invention
The invention relates to a method for elimination of the gap which may occur between any two slightly misaligned and/or magnified adjacent exposure fields in the photolithographic processes commonly used in the fabrication of semiconductor integrated circuits, and more particularly to a method of design of the photolithographic pattern masks used in the fabrication of integrated circuit devices which eliminates the gap between any two slightly misaligned adjacent exposure fields employed in the device fabrication process which could result in manufacturing defects in such devices.
(2) Description of the Prior Art
In the fabrication of semiconductor integrated circuit devices, materials which comprise the devices must be formed into patterns. This is accomplished by placement of a photosensitive material called a photoresist on the surface of the material and exposing the photoresist to radiation through a pattern called a photomask or reticle. Subsequent chemical development of the exposed pattern provides a mask for both selective etching and selective ion implantation of the underlying material into the corresponding pattern.The complete group of patterns required to form an integrated circuit device is known as a chip, or, when separated as individual sections of semiconductor, as a die. In manufacturing practice, arrays of circuit chips are located on the semiconductor substrate as closely as practicable. In order to do this, the patterns are produced by sequential projection of the photomask images onto the semiconductor surface to form each device chip, and adjacent fields are formed by repetitive image projection after moving the substrate at each step. A field is the exposure area which contains one or more chips. This method of step-and-repeat formations of the device chip images places great emphasis on the ability to align each pattern accurately to those previously fabricated. The most significant alignment accuracy and/or stepper magnification function comes about from the use of various optical techniques to locate and place the image of the reticle before exposure; the term stepper refers collectively to the optical and mechanical systems used to perform these functions.
If the alignment of the field and/or stepper magnification function is not within the desired tolerances for device fabrication, there can exist a separation between adjacent fields where the exposure areas are supposed to overlap and provide complete exposure of the photoresist. Such a separation is known as a field gap, and it can lead to there being a region where the photoresist is non-exposed altogether. This in turn can result in pieces or strings of unexposed photoresist being present on the device surface in the case where a positive-working photoresist is being used. These pieces of photoresist constitute manufacturing defects which are costly due to the potential decrease in yield, the need for increased visual inspection, and the potential for subsequent device functional or reliability problems.
The alignment field gap problems described arise due to the consequences of slight alignment errors at the peripheral regions of any two adjacent fields. In another aspect of integrated circuit device fabrication, a clear region is left as a border around each chip to allow clean separation of individual circuit chips or dies after fabrication is complete. Such clear border regions are known as scribe lines from the method of scribing employed in die separation. Scribe line regions must be kept free of any material or residue which might interfere with clean die separation, and a considerable knowledge and art exists concerning such scribe lines, their formation, and their manipulation, particularly with respect to the concerns over alignment, freedom from stray photoresist residues, etc. For example, U.S. Pat. No. 5,290,711 to Yanagisawa describes the prevention of photoresist formation in the scribe line regions of adjacent device chips, but does not discuss the problem of inadvertent misalignment leading to undesired photoresist exposure, or lack thereof, which might cause the formation of undesired photoresist residues.